This invention relates to a packing for a stuffing box and more particularly a graphite derived material which forms a strip seal around a rotatable shaft located therein.
A stuffing box is an annular chamber around a shaft or a plunger, defined on the inside by the surface of the shaft and on the outside by a coaxial, cylindrical casing wall. One axial end is substantially closed by a casing wall while the other end is open to receive a so-called gland. Sealing is effected by a packing of soft or at least yieldable material with which the stuffing box is filled and which is compressed by the gland, pressed axially into the stuffing box, so that the packing bears radially and sealingly upon the shaft surface. To this end, it is important that the packing has resilient properties in order to yield to slight radial motions of the shaft or of the plunger without losing contact therewith. It is therefore common practice to construct stuffing box packings from fibrous materials.
Because of the sealing requirement, graphite has been used in the past as the strip seal material.
Graphite strip seals have been of two general types. One of these types is in the form of compressed rings. These rings are made of conventionally processed crystalline graphite and has a high modulus of elasticity and, as a result, it is necessary for the stuffing box gland to apply high axial forces to these rings to insure the required radial contact. Consequently, the rings have essentially no further yield to radial motions of the shaft. Another difficulty is that the rings must be precisely shaped for specific fitting dimensions, and are therefore very costly.
It is also known to coil graphite packing from a strip seal which has a looser structure than conventional crystalline graphite. This material has a density of approximately 1.0 g/cm.sup.3 while crystalline graphite has a density of approximately 2.25/cm.sup.3. Its modulus of elasticity is less than 1000 N/mm.sup.2, while normal crystalline graphite has a modulus of elasticity of approximately 9000 N/mm.sup.2. Graphite is a brittle material. To enable a graphite strip nevertheless to be coiled, it is provided with a plurality of small grooves extending transversely to the longitudinal axis of the strip. Owing to the loose structure, a stuffing box packing produced from this material is more resilient than a packing of press-formed graphite rings; however, an even higher resilience would be desirable.
The known strip seal also suffers from the disadvantage of calling for high axial pressure to ensure reasonable uniform contact between the stuffing box packing and the surface of the shaft or plunger. It has been found that the stuffing box rings coiled from the aforementioned seal configuration offer a high resistance to axial force. If the axial force exceeds the resistance of the packing, the individual layers or strata thereof break uncontrollably within the packing and slide into each other, thus leading to an irregular distribution of sealing material within the cavity.
The prior art also discloses ring seals coiled from metal foil (German Pat. No. 434,041) in which the cross-section of the metal foil strips is corrugated. The strip has the same corrugation cross-section throughout so that the corrugations of the metal foil strips, which are placed one upon the other in the coiled ring, bear completely upon each other. If axial pressure is applied to this ring, the corrugations curve and simultaneously expand radially. Since the foil strips bear upon each other without any cavities and thus form a solid metal member, it follows that the ring is unable to yield to radial motions of the shaft or of the plunger. It is therefore inelastic. Accordingly, it is employed only for static purposes as a flat seal and is not used for moving shafts or plungers. It is also known from U.S. Pat. No. 3,378,432 and 3,956,543 cited during prosecution of the present application to provide plastics sheeting with a plurality of protuberances and indentations so as to render such sheeting deformable along its plane. However, this is unrelated to strip seals for stuffing box packings.